Answer:
C. The Goldilocks principle
Explanation:
It's named after the Three Bears story, wherein Goldilocks tastes the mama bear's porridge and the dad's, says they're too hot and too cold, respectively, then tries the baby's and says "just right!"
Venus is much too hot for humans to live on.
Mars is too cold for us to live on (right now, that is!!).
Earth is "just right!"